Microminiature field emitters are well known in the microelectronics art. These microminiature field emitters are finding widespread use as electron sources in microelectronic devices. For example, field emitters may be used as a source of electrons in electron guns employed in flat panel displays for use in aviation, automobiles, workstations, laptop computers, head mounted displays, head-up displays, outdoor signage, or practically any application for a screen which conveys information through light emission.
When used in a display, the electrons emitted by a field emitter are directed to a cathodoluminescent material. These display devices are commonly called Field Emission Displays (FEDs). A field emitter used in a display may include a microelectronic emission surface, also referred to as a "tip" or "microtip". An extraction electrode or "gate" may be provided adjacent, but not touching, the field emission tip, to provide a field emission gap therebetween. Upon application of an appropriate voltage between the emitting electrode and the gate, quantum mechanical tunneling, or other known phenomena, cause the tip to emit electrons. The emitted electrons are then accelerated towards the anode. In microelectronic applications, an array of field emission tips may be formed on the horizontal face of a substrate such as a silicon semiconductor substrate. Emitting electrodes, gates and other electrodes may also be provided on or in the substrate as necessary. Support circuitry may also be fabricated on or in the substrate.
The electrical theory underlying the operation of an FED is similar to that for a conventional CRT. Electrons emitted from the tips are accelerated by the gate and anode in the direction of the display surface. These high energy electrons strike phosphors on the inside of the display and excite them to luminesce. The phosphor targets may be arranged in pixels to facilitate the formation of an image. An image is produced by the pattern of luminescing phosphor pixels as viewed by an observer on the display screen. This process is a very efficient way of generating a lighted image.
In a CRT, one electron gun for monochrome or three electron guns for color are provided to generate all of the electrons which impinge on the display screen. A complicated deflection device, usually comprising high power electromagnets, is required in a CRT to direct the electron stream towards the desired screen pixels. The combination of the electron gun and deflection device behind the screen necessarily make a CRT display prohibitively bulky.
FEDs, on the other hand, are relatively thin. Each pixel of an FED has its own electron source, typically an array or group of emitting microtips. The high electric field between the cathode and the gate causes electrons to be emitted from the microtips. FEDs are thin because the microtips, and gates, which are the equivalent of an electron gun in a CRT, are extremely small. Further, an FED does not require a deflection device, because each pixel has its own electron gun (i.e. gate and emitters) positioned directly behind it. The emitters need only be capable of emitting electrons in a direction generally normal to the FED substrate and towards the anode.
With reference to FIG. 1, a cross-section of the edge of a typical FED 10 is illustrated. The FED may include a lower field emitter panel 100, and an upper display panel 200. The field emitter panel 100 may include a glass substrate 110 on which field emitter groups 120 are formed. Each field emitter group 120 may include tens, hundreds, or even thousands of individual emitter tips 122.
The upper display panel 200 may include a glass substrate 210 on which a multiplicity of phosphor groups 220 are formed. Each phosphor group may include many individual phosphor grains 222 which luminesce when electrons from the emitter groups 120 strike them. Each phosphor group 220 may correspond to a pixel in the FED 10. The emitter groups 120 and the gate 130 constitute the "electron guns" which shoot streams of electrons towards the pixels, causing them to fluoresce. The electrons may be made to bombard the phosphor groups 220 by providing the upper display panel 200 with a highly positively charged anode 230. Typically the anode 230 may be provided by a thin layer of metal over (or optionally by a transparent conductor layer 235 under) the phosphor particle groups 220. The anode 230 may be maintained at a potential hundreds or thousands of volts above that of the field emitter groups 120.
The lower field emitter panel 100 and the upper display panel 200 may be connected to each other around their respective perimeters by a side spacer 300 through a glass frit 310 which is adhered to the lower panel 100 and the upper panel 200. The inner side of the glass frit 310 may be coated with a getter layer 320. The getter layer 320 may be used to capture gas molecules which may be present within the FED. The getter layer 320, and its relevance to the present invention are explained in greater detail below.
With reference to FIG. 2a, a plan view of several emitter groups 120 are shown as arranged on the lower field emitter panel 100. The cross section A--A identified in FIG. 2a can be viewed in detail in FIG. 1. Each emitter group 120 includes a multiplicity of individual emitter tips 122. Each group 120 may contain hundreds or even thousands of individual emitter tips 122. Only nine emitters are shown per group in FIG. 2a for ease of illustration. The field emitter groups 120 may be arranged in parallel rows, with one gate line 130 serving each row of emitter groups. In between the gate lines 130 there is a gap 140. The getter layer 320 is present only along the inside wall of the glass frit 310.
With reference to FIG. 2b, a plan view of several phosphor groups 220 formed on the inner surface of the upper display panel 200 is shown. The anode which lies over or under the phosphor groups 220 is not shown in this Figure. Each of the phosphor groups 220 may correspond to one of the field emitter groups 120 shown in FIG. 2a. The area between the phosphor groups 220 may comprise glass substrate 210 (shown) or the anode (not shown). The area between the phosphor groups 220 may form a grid or matrix 240 depending on the arrangement of the phosphor groups 220 on the glass substrate 210. The getter layer 320 is present only along the inside wall of the glass frit 310.
With renewed reference to FIG. 1, in order to operate a display, the space between the lower field emitter panel 100 and the upper display panel 200 should be evacuated. Typically, this space may be of the order of a 1 millimeter separation. As noted above, the glass substrate 110 underlying the field emitter groups 120 and the glass substrate 210 supporting the phosphor groups 220 may be sealed to one another along their respective edges with the glass frit 310, encompassing a spacer 300. After being sealed, the space between the two glass substrates, 110 and 210, may be evacuated of gas and sealed off from the outside atmosphere.
Residual gas on, in, or above the surfaces of two glass substrates, 110 and 210, can increase the probability of electric flash-overs. It is very common for residual gas to be absorbed into the metal or other interior surfaces of an FED during processing. Once the interior of the FED is evacuated, these absorbed gases tend to outgas into the interior of the FED. A residual gas molecule may typically adhere to an interior surface of the FED, float away until it strikes another surface, adhere to the new surface for a while, etc. Because the interior space of an FED is a relatively long narrow space, the gas molecules, depending on the mean free path, collide with the walls and between themselves with a maxwellian distribution of velocities. During this random movement, some of the molecules may arrive at the perimeter of the FED panel and strike the surface of the getter. As the getter acts as a chemical pump, the local pressure in the vicinity of the getter surface may fall. This may set up a pressure gradient between the bulk of the space in the FED and the space close to the getter surface. Thus a directed flow of gas molecules towards the getter takes place.
If the getter capacity is limited, there may arise a situation of net increase in the population of residual gas molecules in the panel space and the gas population may further increase due to the desorption of gas molecules from the surfaces of interior structures.
The accumulated gas molecules in the FED may become easily ionized due to the high energy electrons within the FED. With continued reference to FIG. 1, the ionized gas molecules may provide an electrical path for flash-overs between adjacent gate lines 130, between emitter tips 122 and gates lines 130, and even between gate lines 130 and the anode 230. Flash-overs can damage or destroy an FED. In FEDs in which the potential between the anode 230 and the gate lines 130 is in the range of thousands of volts, flash-over may be catastrophic to the device 10. Therefore, it is imperative to reduce the amount of residual gases within the FED as much as possible. Even if a flash-over is not initially catastrophic, it may result in overheating of the materials within the FED, resulting in the release of additional gas molecules thereby enhancing the probability of future flash-over.
One method of addressing the residual gas problem in displays has been to capture the gas in a getter located within the display. CRTs typically include a getter consisting of a wire or ring of chemically reactive metals covered with a passivation layer of a material than can be thermally disrupted to expose the chemically reactive material after the display has been assembled and evacuated.
Jones, U.S. Pat. No. 5,534,743 (Jul. 9, 1996) for Field Emission Display Devices, and Field Emission Electron Beam Source and Isolation Structure Components Therefor, herein incorporated by reference, discloses a getter arrangement for use in an FED. The '743 patent discloses a flat panel display assembly having an extension portion defining an extension volume in which a getter capsule containing an active getter may be disposed. The getter may be chemisorptively effective for removal of gases in the interior volume of the display.
Previous attempts to control flash-over by capturing gas within an FED have consisted of placing a layer of getter material along the inside of the outer perimeter wall joining the two flat panels of the display. With continued reference to FIG. 1, a getter 320 may be provided along the outer perimeters of the glass substrates, 110 and 210, and/or along the inside of the glass frit 310. A resistive heating element 330 may be provided under the getter 320, and a protective coating 340 provided over the getter. An example of a known getter is described in an article entitled "An updated review of getters and gettering" by T. A. Girogi et al., published in J. Vac. Sci. Technol. A 3(2). (March/April, 1985).
Space requirements have largely dictated the location of the getter material. With reference to FIG. 3a, a resistive heating element 330 may be provided along the upper or lower substrate 110 or 210. The getter 320 may be provided over the resistive heating element 330, and a protective coating 340 is provided over the getter. The getter and the protective coating may be applied under vacuum, so that the getter does not come into contact with any gas before being sealed by the protective coating. After the FED has been evacuated and sealed, the resistive heating element 330 is heated. With reference to FIG. 3b, the heat from the resistive heating element causes the protective coating 340 to melt (and not the getter 320) and at least partially exposes the getter 320 to the vacuum within the FED. As gas molecules are released from the internal surfaces of the FED over time, the getter 320 may be able to absorb the gas and prevent flash-overs.
As noted above, released gas molecules may spend the majority of their time in the FED and may adhere to an inner surface of the FED. The probability of flash-over may be greatly reduced by reducing the residual gas molecules on the inner FED surfaces. Residual gas can support pre-ionization under high voltages and eventually lead to arc discharge (flash-over). Minimizing the residual gas molecules, and thus preventing flash-overs (arcs), is difficult in a FED because of the high fields existing at the sharp tips and the gates.
The gases desorbed from the interior FED surfaces wander about at random with speeds characterized by the temperature of the gas. As stated previously, the gas molecules collide among themselves and with the walls in which they are contained. If a getter is located at the perimeter of the display, the chemi-sorption of gases at the getter site results in a directed flow of gases from the center of the panel space to the getter site.
The number of collisions the gas molecules make with the walls of the interior FED surfaces will be characterized by the magnitude of the mean free path in relation to the dimensions of the structures. Every time a gas molecule collides with a wall, the chances of its retention at the wall (physi-sorption) depends on the sticking coefficient. The sticking coefficient may vary with the nature of the gas species and the nature of the material with which the gas molecule collides. Obviously the sticking coefficient will vary for the surfaces of a microtip, gate metal, phosphor, etc. Before the gas molecules find their way to the getter, a number of complex collision mechanisms occur. These mechanisms dictate the time spent by the gas molecules in the panel space before they arrive at the getter site. Obviously, the larger the panel size, the longer the transit time of the gas molecules to the getter site. It may take a week to establish the equilibrium pressure at which the residual gas is drawn to the perimeter getter. If the panel is operated before this equilibrium is reached, flash-overs may become imminent. To reduce this long transit time, Applicants have developed "on-site gettering" inside the panel. This getter for the gas molecules is provided in the region that the gas molecules are desorbed instead of being provided at the perimeter of the panel where the gas must drift to the getter. This leads to an improved apparatus and method of "on-site gettering" of residual gases inside an FED which may reduce the occurrence of flash-overs.